Photocatalyst and method for producing the same

Abstract

To provide a photocatalyst using a ferrite-based material that exhibits good activity, and a method for producing the same. [Solution] The photocatalyst is a photocatalyst comprising a polycrystalline ferrite material, wherein the polycrystalline ferrite material has a coelectric field Ec of 0.1 MV / m or more and 2.0 MV / m or less.

Description

[Technical Field]

[0001] This disclosure relates to photocatalysts and methods for producing the same. [Background technology]

[0002] Ferrite materials are inexpensive and readily available because they are composed mostly of iron and oxygen, and because they are oxides, they have excellent chemical stability and are resistant to corrosion. For this reason, they are very desirable materials for practical use. As ferrite materials for photocatalysis, composite materials are being investigated that combine bismuth ferrite (BiFeO3), which has a perovskite structure, with other materials (Patent Documents 1-4). In addition, the use of polarized bismuth ferrite as a photocatalyst is being investigated (Non-Patent Document 1). [Prior art documents] [Patent Documents]

[0003] [Patent Document 1] Chinese Patent Application Publication No. 104646001 Specification [Patent Document 2] Chinese Patent Application Publication No. 104941662 Specification [Patent Document 3] Chinese Patent Application Publication No. 106807400 Specification [Patent Document 4] Chinese Patent Application Publication No. 108114736 Specification [Non-patent literature]

[0004] [Non-Patent Document 1] Journal of Alloys and Compounds 783(2019)943-951 [Overview of the project] [Problems that the invention aims to solve]

[0005] Ferrite materials are useful because they can be synthesized using materials that are readily available industrially. However, the photocatalytic activity of ferrite materials needs further improvement.

[0006] The object of this disclosure is to provide a photocatalyst using a ferrite-based material that exhibits good activity, and a method for producing the same. [Means for solving the problem]

[0007] The inventors of this disclosure have found that a polycrystalline ferrite material having a specific coelectric field Ec can be obtained by performing a poling treatment (polarization treatment) on a ferrite material after annealing. They have confirmed that this polycrystalline ferrite material having a specific coelectric field Ec exhibits good activity as a photocatalyst, and have completed the embodiments of this disclosure. Accordingly, the embodiments of this disclosure are as follows.

[0008] [1] A photocatalyst comprising a polycrystalline ferrite material, wherein the polycrystalline ferrite material has a coelectric field Ec of 0.1 MV / m or more and 2.0 MV / m or less. [2] The photocatalyst according to [1], wherein the polycrystalline ferrite material has a perovskite crystal structure. [3] The photocatalyst according to [1] or [2], wherein the polycrystalline ferrite material is a nanoparticle. [4] The photocatalyst according to [1] or [2], wherein the polycrystalline ferrite material is a fiber.

[0009] [5] A method for producing a photocatalyst, comprising: a first step of preparing a polycrystalline ferrite material; a second step of annealing the polycrystalline ferrite material; and a third step of applying an electric field to the polycrystalline ferrite material. [6] The method for applying the electric field in the second step is a corona discharge treatment method, the method for producing a photocatalyst as described in [5]. [7] The polycrystalline ferrite material is nanoparticles, and the first step is a step of producing the nanoparticles of the polycrystalline ferrite material by a solvothermal method, the method for producing a photocatalyst according to [5] or [6]. [8] The polycrystalline ferrite material is nanofibers, and the first step is a step of producing the nanofibers of the polycrystalline ferrite material by an electrospinning method, the method for producing a photocatalyst according to [5] or [6].

Advantages of the Invention

[0010] According to the present disclosure, it is possible to provide a photocatalyst using a ferrite-based material, which exhibits good activity, and a method for producing the same.

Brief Description of the Drawings

[0011] [Figure 1] XRD chart of the nanoparticles (BiFeO3) obtained in Example 1. [Figure 2] XRD chart of the nanofibers (BiFeO3) obtained in Example 2. [Figure 3] SEM image of the nanoparticles (BiFeO3) obtained in Example 1. [Figure 4] SEM image of the nanofibers (BiFeO3) obtained in Example 2. [Figure 5] Hysteresis loop of the nanofibers (BiFeO3) obtained in Example 2. [Figure 6] Graph showing the results of the photodegradation experiment of methylene blue using the nanoparticles before poling treatment and the nanoparticles after poling treatment obtained in Example 1. [Figure 7] Graph showing the results of the photodegradation experiment of indigo carmine using the nanoparticles before poling treatment and the nanoparticles after poling treatment obtained in Example 1. [Figure 8] Graph showing the results of the photodegradation experiment of indigo carmine using the nanofibers before poling treatment and the nanofibers after poling treatment obtained in Example

Mode for Carrying Out the Invention

[0012] (Photocatalyst material) The photocatalyst according to an embodiment of the present disclosure includes a polycrystalline ferrite material. The polycrystalline ferrite material may have any crystal structure of a perovskite structure, a spinel crystal structure, a hexagonal crystal structure, or a garnet crystal structure. As the polycrystalline ferrite material having a perovskite structure, those represented by MFeO3 (where M represents at least one rare metal selected from the group consisting of La and Bi) can be used. As the polycrystalline ferrite material having a spinel crystal structure, M 12 , 19 ,

[0014] , II , , II , , 12 , , ,

[0013] Fe2O4 (where M I represents at least one magnetic metal selected from the group consisting of Fe, Ni, and Co) can be used. As the polycrystalline ferrite material having a hexagonal crystal structure, M II Fe 12 O 19 (where M II represents at least one metal selected from the group consisting of Ba, Sr, and Pb) can be used. As the polycrystalline ferrite material having a garnet crystal structure, RFe5O 12 (where R is a rare earth element) can be used. <​​​​​​Grain boundaries may exist between the crystals of the polycrystalline ferrite material. These grain boundaries may be small enough to be visually detectable in SEM (scanning electron microscope) images.

[0015] There are no particular restrictions on the shape of the polycrystalline ferrite material; it may be nanoparticles or nanofibers. Nanoparticles refer to particles with a diameter of 1000 nm or less, while nanofibers refer to fibrous materials with a diameter between 1 nm and 1000 nm and an aspect ratio of 100 or more.

[0016] (Shape of nanoparticles) The nanoparticles of the polycrystalline ferrite material may be spherical, rod-shaped, flake-shaped, flower-shaped, amorphous, etc., and may include any combination of two or more of these shapes. The average aspect ratio (average major axis / average minor axis) of rod-shaped nanoparticles may be less than 100. The ratio of the average major axis to the average thickness (average major axis / average thickness) of flake-shaped nanoparticles may be 100 or more.

[0017] There are no particular limitations on the size of the nanoparticles; for example, they may have an average particle diameter of 1 to 1000 nm. The average particle diameter of the nanoparticles refers to the arithmetic mean of the major and minor axes of 10,000 nanoparticles. The major and minor axes of the nanoparticles are values ​​measured using SEM images.

[0018] The nanoparticles may form secondary particles (aggregates). There are no particular limitations on the size of the secondary particles. For example, the particle diameter of the secondary particles may be 1 μm or larger, or 10 μm or smaller. The particle diameter of the secondary particles is a value measured using SEM imaging.

[0019] (Shape of nanofibers) Nanofibers of polycrystalline ferrite material may exhibit pores and / or cavities as observed by surface observation using a scanning electron microscope (SEM).

[0020] There are no particular limitations on the size of the nanofibers; for example, they may have an average diameter of 1 to 1000 nm and an average length greater than 1000 nm. The average aspect ratio (average length / average diameter) of the nanofibers may be 100 or greater. The average length and average diameter of the nanofibers represent the arithmetic mean of the length and diameter of 10,000 nanofibers. The length and diameter of the nanofibers are values ​​measured using SEM images.

[0021] (Dielectric properties of polycrystalline ferrite materials) The polycrystalline ferrite material has been subjected to a polling treatment (polarization treatment). The polycrystalline ferrite material has been polled to have a coelectric field Ec of 0.1 MV / m or more and 2.0 MV / m or less. The coelectric field Ec may be in the range of 0.2 MV / m or more and 1.9 MV / m or in the range of 0.4 MV / m or more and 1.7 MV / m or less. Since the coelectric field Ec is high, above 0.1 MV / m, it promotes the generation of excited species such as hydroxyl radicals under light irradiation, resulting in high catalytic activity. In addition, since the material can be polled with a coelectric field Ec of 2.0 MV / m or less, degradation during the polling treatment can be suppressed.

[0022] (Crystallization of polycrystalline ferrite materials) The polycrystalline ferrite material may be an annealed product. Annealed polycrystalline ferrite material has higher crystallinity compared to the material before annealing. This increased crystallinity allows for stable polarization by poling treatment, and enhances the catalytic activity of the polycrystalline ferrite material.

[0023] (Supporting of co-catalysts) Polycrystalline ferrite materials may have co-catalyst particles supported on their surface. As co-catalyst particles, for example, particles of a metal or metal oxide with a Fermi level lower than that of the polycrystalline ferrite material can be used. By supporting these co-catalyst particles on the polycrystalline ferrite material, the generation of hydroxyl radicals upon light irradiation is promoted, thereby improving the photocatalytic activity. In the case of polycrystalline bismuth ferrite, the co-catalyst particles may be, for example, one selected from the group consisting of Au, Pd, Ag, and Pt, or particles of an alloy combining two or more of these metals. The surface of the co-catalyst particles may be covered with an oxide such as silica. Such a structure is called a core-shell structure. Examples of co-catalyst particles with a core-shell structure are Au core-silica shell particles and Ag core-silica shell particles.

[0024] There are no particular limitations on the size of the co-catalyst particles; for example, they may have an average particle diameter of 1 to 1000 nm. If the polycrystalline ferrite material is nanoparticles, the average particle diameter of the co-catalyst particles may be smaller than the average particle diameter of the nanoparticles of the polycrystalline ferrite material. If the polycrystalline ferrite material is nanofibers, the average particle diameter of the co-catalyst particles may be smaller than the average diameter of the nanofibers of the polycrystalline ferrite material. The average particle diameter of the co-catalyst particles represents the arithmetic mean of the major and minor axes of 10,000 co-catalyst particles. The major and minor axes of the co-catalyst particles are values ​​measured using SEM images.

[0025] There are no particular limitations on the amount of co-catalyst particles supported. For example, the amount of co-catalyst particles supported relative to the total mass of the polycrystalline ferrite material and co-catalyst particles may be within the range of 0.2% by mass to 10% by mass.

[0026] (Band gap) The polycrystalline ferrite material (nanoparticles or nanofibers) according to this embodiment may have a band gap of 1.0 eV to 3.5 eV.

[0027] (Mechanism of action) When light such as ultraviolet light or visible light is irradiated onto the photocatalyst (nanops or nanofibers of polycrystalline ferrite material) of this embodiment, electrons are ejected from its surface, generating holes. These holes react with moisture on the surface to produce excited species such as hydroxyl radicals, which have strong oxidizing power. These excited species can decompose pollutants such as organic compounds that make up dirt.

[0028] The photocatalyst in this embodiment may be used in the form of nanoparticles or nanofibers. Alternatively, the photocatalyst may be used fixed to the surface of a substrate. Suitable substrate materials include, for example, metal, tile, enamel, cement, concrete, glass, fiber, wood, paper, plastic, etc. Conventional methods known as catalyst fixation methods, such as sintering the photocatalyst or using a binder, can be used to fix the photocatalyst to the substrate. Furthermore, the photocatalyst may be used in the form of a molded body. Nanoparticles may be molded into shapes such as flat plates, corrugated plates, honeycomb structures, spheres, or curved surfaces by adding a binder as needed. Nanofibers may be, for example, in the form of a nonwoven fabric in which they are intertwined.

[0029] (Method for manufacturing photocatalysts) A method for producing a photocatalyst according to one embodiment of the present disclosure comprises a first step of preparing a polycrystalline ferrite material, a second step of annealing the prepared polycrystalline ferrite material, and a third step of applying an electric field to the annealed polycrystalline ferrite material. The polycrystalline ferrite material prepared in the first step may be nanoparticles or nanofibers.

[0030] (Step 1 - Method for synthesizing nanoparticles) Methods for synthesizing nanoparticles of polycrystalline ferrite material include the breakdown process and the build-up process. The breakdown process is a process in which coarse particles or lumps of polycrystalline ferrite material are mechanically crushed using a grinding device to obtain fine, small nanoparticles. Examples of grinding devices include jet mills, ball mills, planetary mills, and bead mills.

[0031] The build-up process is a process that obtains nanoparticles from polycrystalline ferrite raw materials using physical and chemical reactions. Synthesis methods such as gas-phase, liquid-phase, and solid-phase methods can be used to obtain nanoparticles. Examples of gas-phase methods include chemical vapor deposition (CVD) and physical vapor deposition (PVD). CVD methods include thermal CVD, plasma CVD, and flame CVD. Examples of liquid-phase methods include irradiation methods that irradiate the raw materials with physical energy, sol-gel methods, liquid-phase reduction methods, and solvothermal methods. Irradiation methods include spray pyrolysis, laser decomposition, and ultrasonic methods. Examples of solid-phase methods include solid-phase pyrolysis.

[0032] The solvothermal method is preferred for synthesizing nanoparticles. In the solvothermal method, polycrystalline ferrite raw materials are reacted in a solvent under high temperature and high pressure or a supercritical state. Water, organic solvents (e.g., ethylene glycol), and mixed solvents thereof can be used as solvents.

[0033] (Step 1 - Method for synthesizing nanofibers) Electrospinning can be used as a method for synthesizing nanofibers of polycrystalline ferrite material. The synthesis of nanofibers of polycrystalline ferrite material by electrospinning is carried out, for example, as follows: A raw material solution containing a precursor of polycrystalline ferrite material and a binder resin is placed in a syringe. Then, while applying an electric field to the raw material solution in the syringe, the raw material solution is extruded to the outside in a fibrous form to spin primary fibers. Primary fibers can be obtained by maintaining a constant ambient temperature and humidity during spinning. Next, the obtained primary fibers are dried. A constant-temperature oven can be used as the drying apparatus. For example, polyvinylpyrrolidone can be used as the binder resin in the raw material solution. The density of the nanofibers and the proportion of surface pores and / or cavities can be adjusted by changing the amount of binder resin in the raw material solution.

[0034] (Step 2) In the second step, the polycrystalline ferrite material prepared in the first step is annealed. The annealing temperature is in a range that is above the temperature at which the crystallinity of the polycrystalline ferrite material improves, and below the temperature at which the polycrystalline ferrite material sintersects or melts. In the case of polycrystalline bismuth ferrite, the annealing temperature is, for example, in the range of 500°C to 1000°C. The annealing environment may be, for example, an atmospheric environment. The annealing time may be, for example, 30 minutes to 5 hours. A muffle furnace can be used as the heating device.

[0035] (Step 3) In the third step, the polycrystalline ferrite material annealed in the second step is subjected to a poling treatment by applying an electric field. The method of applying the electric field is not particularly limited, but examples include a method in which the polycrystalline ferrite material is placed on a metal electrode and then the electric field is applied by corona discharge treatment, or a method in which an acrylic monomer solution containing the polycrystalline ferrite material is cast onto a metal, the acrylic monomer is polymerized, and then a metal foil is placed on top and the electric field is applied.

[0036] The applied electric field is not particularly limited, but it is preferable to apply a voltage of 100 kV / cm or higher. The applied voltage may be in the range of 200 kV / cm to 2000 kV / cm, or in the range of 500 kV / cm to 1500 kV / cm.

[0037] As described above, a polycrystalline ferrite material having the aforementioned dielectric properties can be obtained.

[0038] (Supporting of co-catalysts) When a co-catalyst is supported on the surface of a polycrystalline ferrite material, the step of supporting the co-catalyst may be performed at any time between the first and second steps, between the second and third steps, or after the third step.

[0039] To support metal particles, for example, a polycrystalline ferrite material can be dispersed in a solvent containing a dissolved metal salt, and then a reducing agent can be added to the solution to precipitate the metal particles onto the surface of the polycrystalline ferrite material. An example of a reducing agent is ethanol.

[0040] When supporting metal oxide particles, for example, a polycrystalline ferrite material can be dispersed in a solvent containing an organometallic compound such as TEOS, and then the metal oxide particles can be deposited on the surface of the polycrystalline ferrite material by adjusting the pH of the solution or by heating and drying.

[0041] (Methods for breaking down pollutants) The photocatalyst of this embodiment can be used as a photocatalyst for decomposing pollutants. Next, a method for decomposing pollutants will be explained using contaminated water containing pollutants as an example.

[0042] The method for decomposing pollutants using a photocatalyst according to this embodiment comprises the steps of: bringing contaminated water containing pollutants into contact with the photocatalyst under a light-irradiated environment to decompose the pollutants; and separating the contaminated water from the photocatalyst. The photocatalyst may be in the form of nanoparticles or nanofibers, fixed to the surface of a substrate, or in the form of a molded body.

[0043] If the photocatalyst is in the form of nanoparticles or nanofibers, the photocatalyst is dispersed in the contaminated water, the pollutants are decomposed under light irradiation, and then the photocatalyst is separated from the contaminated water. Various methods used for solid-liquid separation, such as filtration, decantation, and centrifugation, can be used to separate the photocatalyst from the contaminated water. Since nanoparticles or nanofibers have a large surface area, the efficiency of decomposing pollutants is high. If the polycrystalline ferrite material used as the photocatalyst is ferromagnetic, magnetic force may be used to separate it, such as by attracting it with a magnet. By using magnetic force, fine polycrystalline ferrite material can be separated and recovered with high efficiency, so the amount of photocatalyst mixed into the treated contaminated water is reduced. This reduces the burden on the environment.

[0044] If the photocatalyst is fixed to the surface of a substrate or in the form of a molded body, the photocatalyst is brought into contact with contaminated water, the pollutants are decomposed under light irradiation, and then the photocatalyst is separated from the contaminated water. When the photocatalyst is fixed to the surface of a substrate or in the form of a molded body, the separation of the photocatalyst from the contaminated water becomes easier. If the photocatalyst is a nonwoven fabric, the decomposition of pollutants and the separation of the photocatalyst from the contaminated water can be continuously performed by passing contaminated water through the nonwoven fabric under light irradiation.

[0045] (Information processing device) This disclosure also relates to an information processing device. The information processing device of this embodiment transmits at least photocatalyst information relating to a photocatalyst, ferroelectricity-related information relating to the coercive field Ec, and photocatalyst activity-related information indicating photocatalyst activity based on the relationship between the photocatalyst-related information and the ferroelectricity-related information. This allows us to accurately communicate the technical significance of photocatalysts that utilize ferroelectricity to users of photocatalysts. [Examples]

[0046] (Medications used) Bismuth nitrate (Bi(NO3)3·5H2O, ≥98%), iron nitrate (Fe(NO3)3·9H2O, ≥98%), nickel sulfate (NiSO4·6H2O, ≥98%), nitric acid (HNO3, 65%), and memalachite green oxalate (MG, ≥90%) were all obtained from Sigma-Aldrich. Potassium hydroxide (KOH, 85%) and urea (99.5%) were purchased from Kanto Chemical Co., Ltd. All reagents were used as received. Deionized water (Milipore System, 18.2Ω) was used as the solvent.

[0047] (Example 1) Preparation of bismuth ferrite nanoparticles 2.43 g of Bi(NO3)3·5H2O, 2.02 g of Fe(NO3)3·9H2O, 0.3 g of urea, and 2 mL of HNO3 were mixed, and deionized water was added to make a total volume of 20 mL. The mixture was continuously stirred until all chemicals were completely dissolved. Subsequently, the homogenized solution was mixed with 60 mL of 14 M KOH aqueous solution. The resulting mixture was transferred to an autoclave reactor coated with polytetrafluoroethylene and heated at 180 °C for 24 hours. After cooling to room temperature, the resulting product was centrifuged, washed with ethanol and deionized water, and finally dried in an oven at 80 °C to obtain nanoparticles. The obtained nanoparticles were thermal annealed in a muffle furnace at 600 °C for 2 hours in an air atmosphere. The annealed nanoparticles were subjected to poling by applying an electric field by corona discharge at an applied voltage of 1000 kV / cm.

[0048] (Example 2) Preparation of bismuth ferrite nanofibers A bismuth ferrite sol-gel solution was prepared by dissolving 4.0 g of Bi(NO3)3·5H2O and 3.03 g of Fe(NO3)3·9H2O in 10 mL of 2-methoxyethanol. The pH of the sol-gel solution was adjusted to 3.0-4.0 by adding 0.05 mL of ethanolamine. Next, 5 mL of glacial acetic acid was added to adjust the viscosity of the sol-gel solution. Solution A was prepared by stirring the sol-gel solution, whose pH and viscosity had been adjusted, at room temperature for about 2 hours. Solution B, the second solution, was prepared by adding 2 g of polyvinylpyrrolidone (PVP) with a mass-average molecular weight (Mw) of 360,000 to 11 g of a dimethylformamide (DMF) / ethanol (1:1 wt / wt) solvent mixture. The prepared solution B was mechanically stirred for about 1 hour. Next, solution A was added dropwise to solution B under constant stirring conditions to obtain a homogeneous raw material solution for the electrospinning process.

[0049] Primary bismuth ferrite fibers were spun using a nanofiber electrospinning unit (NEU) purchased from Kato Tech Co., Ltd. (Japan) and the resulting raw material solution. The spinning conditions for the electrospinning unit were: applied voltage: 12kV, distance between needle tip and collector: approximately 10cm, and raw material solution supply rate: 0.1mm / min. The primary fibers were dried in a constant temperature oven at 100°C for 1 hour to obtain nanofibers. The obtained nanofibers were thermal annealed in a muffle furnace at 600°C for 2 hours in an air atmosphere. After annealing, the nanofibers were subjected to poling treatment by applying an electric field by corona discharge at an applied voltage of 1000kV / cm.

[0050] [XRD measurement] The polling-treated nanoparticles obtained in Example 1 and the polling-treated nanofibers obtained in Example 2 were subjected to XRD measurements under the following conditions. Figure 1 shows the XRD chart of the polling-treated nanoparticles (BiFeO3) obtained in Example 1, the XRD chart of the nanoparticles before polling, and the standard XRD chart of BiFeO3 (jCPDS card #71-2494). Figure 2 shows the XRD chart of the polling-treated nanofibers (BiFeO3) obtained in Example 2 and the standard XRD chart of BiFeO3 (jCPDS card #71-2494).

[0051] Equipment used: Rigaku MiniFlex Method: 2θ-θ reflection method X-ray used: Cu-Kα ray Scan speed: 1.00° / min Sampling interval: 0.10° Slit width: DS: (variable), SS: 4,2°, RS: 0.3mm

[0052] The XRD chart in Figure 1 clearly shows that the nanoparticles obtained in Example 1 after polling treatment are BiFeO3. Furthermore, a comparison of the XRD chart of the nanoparticles before polling treatment with the standard XRD chart of BiFeO3 revealed that the perovskite structure is stably maintained in the nanoparticles after polling treatment.

[0053] The XRD chart in Figure 2 clearly shows that the nanofibers obtained in Example 2 after polling treatment are BiFeO3.

[0054] [SEM observation] The polling-treated nanoparticles obtained in Example 1 and the polling-treated nanofibers obtained in Example 2 were observed using a scanning electron microscope (SEM). Figure 3 shows the SEM image of the nanoparticles (BiFeO3) obtained in Example 1, and Figure 4 shows the SEM image of the nanofibers (BiFeO3) obtained in Example 2. SEM observation confirmed that the nanoparticles obtained in Example 1 were polycrystalline particles with secondary particle size of approximately 1 to 10 μm, and the primary particle diameter was in the range of 1 to 1000 nm. SEM observation confirmed that the nanofibers obtained in Example 2 had a diameter in the range of 1 to 1000 nm, a length in the range of 1 to 10 μm, and an aspect ratio of 5 or greater.

[0055] [Optical absorption spectrum measurement] The optical absorption spectra of the polling-treated nanoparticles obtained in Example 1 and the polling-treated nanofibers obtained in Example 2 were measured. From the obtained optical absorption spectra, it was confirmed that the band gaps of the nanoparticles obtained in Example 1 and the nanofibers obtained in Example 2 were 2.0 to 2.5 eV.

[0056] [Measurement of coercive field Ec] The ferroelectric properties (hysteresis loops) of the nanofibers obtained in Example 2, after thermal annealing and before poling treatment, were measured using a piezoelectric response force microscope (PFM) equipped with an MFP-3D probe microscope (Asylum Research, Oxford Instruments, UK), and the coercive field Ec was determined from the obtained hysteresis loops. The nanofibers were mounted on the PFM stage using silver paste. Hysteresis loop measurements were performed using a PPP-NCSTPt-20 PFM probe (Nanosensors, Nanoworld AG, Switzerland) in dual AC resonance tracking PMF mode. Hysteresis loops were measured at four locations (Loc. 1-4) on the nanofibers. The results are shown in Figure 5. The coercive field Ec obtained from each hysteresis loop shown in Figure 5 was 0.45 MV / m at Loc. 1, 1.7 MV / m at Loc. 2, 0.7 MV / m at Loc. 3, and 0.6 MV / m at Loc. 4. These results confirm that BiFeO3 subjected to poling after thermal annealing has a cohesive field Ec in the range of 0.45 MV / m to 1.7 MV / m.

[0057] [Confirmation of photocatalytic activity] (Evaluation of photocatalytic performance under light irradiation conditions in the photodegradation of methylene blue (MB)) As a sample, the photocatalytic performance under light irradiation conditions in the photodegradation of methylene blue (MB) was evaluated using nanoparticles obtained in Example 1. 20 mL of MB aqueous solution (concentration: 20 μM) was poured into a quartz tube, and 30 mg of the sample was added to the MB aqueous solution. After addition, it was left to stand in the dark for 120 minutes to achieve adsorption-desorption equilibrium, and then 500 mW / cm² was applied to the MB solution. 2Photodegradation experiments of MB were carried out by irradiating light using a xenon lamp (LCS-100, 94011A, Newport) operating at []. At specified time intervals, 0.2 mL of the MB aqueous solution was collected, centrifuged to remove the solids dispersed in the solution. To determine the MB concentration of the MB aqueous solution after removing the solids, the absorbance of the MB aqueous solution at λ = 664 nm was measured using a UV-vis spectrometer (PD-3000UVe, Apel). The results are shown in Fig. 6. The photodegradation experiments of MB were carried out using the nanoparticles before poling treatment and the nanoparticles after poling treatment.

[0058] In Fig. 6, the horizontal axis is the irradiation time of white light, and the vertical axis is the relative concentration of MB with the MB concentration at the start of the MB photodegradation experiment set to 1.0. From the results in Fig. 6, it can be seen that the nanoparticles after poling treatment have a higher decomposition promotion effect in the light irradiation environment of MB compared to the nanoparticles before poling treatment and are useful as a photocatalyst.

[0059] (Evaluation of photocatalytic performance in the light irradiation environment for the photodegradation of indigo blue (IB)) As samples, the photocatalytic performance in the light irradiation environment for the photodegradation of indigo blue (IB) was evaluated using the nanoparticles obtained in Example 1 and the nanofibers obtained in Example 2. 20 mL of an IB aqueous solution (concentration: 50 μM) was injected into a quartz tube, and 10 mg of the sample was added to the IB aqueous solution. After addition, it was left standing in the dark for 120 minutes to achieve adsorption-desorption equilibrium, and then 500 mW / cm 2IB photodegradation experiments were performed by irradiating with light using a xenon lamp (LCS-100, 94011A, Newport). At specified time intervals, 0.2 mL of IB aqueous solution was collected and centrifuged to remove the solids dispersed in the solution. To determine the IB concentration of the IB aqueous solution from which the solids had been removed, the absorbance of the IB aqueous solution at λ=611 nm was measured using a UV-vis spectrometer (PD-3000UVe, Apel). The results are shown in Figures 7 and 8. Figure 7 shows the results of IB photodegradation experiments using nanoparticles before and after poling treatment obtained in Example 1, and Figure 8 shows the results of IB photodegradation experiments using nanofibers before and after poling treatment obtained in Example 2.

[0060] In Figures 7 and 8, the horizontal axis represents the irradiation time with white light, and the vertical axis represents the relative concentration of IB, with the IB concentration at the start of the IB photodecomposition experiment set to 1.0. From the results in Figures 7 and 8, it can be seen that the nanoparticles and nanofibers after poling treatment have a higher decomposition-promoting effect under IB light irradiation compared to before poling treatment, and the photocatalytic action is improved. In particular, as shown in Figure 7, the decomposition efficiency of nanoparticles after poling treatment under IB light irradiation is improved by 11% compared to nanoparticles before poling treatment. This is because the electrons and holes generated by light irradiation are less likely to recombine in polycrystalline BiFeO3 nanoparticles and nanofibers that have been poling treated, and hydroxyl radicals are more easily generated.

[0061] [Additional Note] The photocatalytic nanoparticles and nanofibers made of polycrystalline ferrite material according to this disclosure can exhibit excellent photocatalytic activity when an electric field is applied, and are also easy to recover, thus contributing to Goal 9 of the United Nations-led Sustainable Development Goals (SDGs). Goal 9: "Build resilient infrastructure, promote inclusive and sustainable industrialization and foster innovation."

[0062] Embodiments of this disclosure are described below.

[0063] [Note 1] A photocatalyst comprising a polycrystalline ferrite material, wherein the polycrystalline ferrite material has a coelectric field Ec. The polycrystalline ferrite material contained in the photocatalyst described in Appendix 1 has a coelectric field Ec and is polarized, so electrons and holes generated by light irradiation do not easily recombine. Therefore, excited species such as hydroxyl radicals are easily generated. Thus, the photocatalyst described in Appendix 1 exhibits good activity.

[0064] [Note 2] The polycrystalline ferrite material has a perovskite crystal structure, as described in Appendix 1. The polycrystalline ferrite material included in the photocatalyst described in Appendix 2 has a perovskite crystal structure, and therefore possesses high polarity and a stable crystal structure. Thus, the photocatalyst described in Appendix 2 exhibits good activity over a longer period of time.

[0065] [Note 3] The polycrystalline ferrite material is an annealed product, as described in Appendix 1 or 2. The polycrystalline ferrite material contained in the photocatalyst described in Appendix 3 is an annealed product, exhibiting high crystallinity and improved polarity. Therefore, the photocatalyst described in Appendix 3 shows good activity over an even longer period.

[0066] [Note 4] The polycrystalline ferrite material is a nanoparticle, and the photocatalyst is one of the photocatalysts described in any one of the appendices 1 to 3. Since the polycrystalline ferrite material contained in the photocatalyst described in appendice 4 is a nanoparticle, it is easy to disperse in liquid and can be molded into various forms. Therefore, the photocatalyst described in appendice 4 can be used for a variety of applications.

[0067] [Note 5] The photocatalyst described in Appendix 4 has a photocatalyst made of polycrystalline ferrite nanoparticles with an average particle size of 1 nm to 1000 nm. Since the average particle size of the nanoparticles contained in the photocatalyst described in Appendix 5 is within the above range, it has a large surface area. Therefore, the photocatalyst described in Appendix 5 exhibits even greater activity.

[0068] [Note 6] The polycrystalline ferrite material is a nanofiber, and is a photocatalyst as described in any one of the appendices 1 to 3. Since the polycrystalline ferrite material contained in the photocatalyst described in appendice 6 is a nanofiber, it can be molded into the shape of a nonwoven fabric, etc. Furthermore, when nanofibers are dispersed in contaminated water, they have a higher recovery efficiency by filters and do not clog filters as easily as nanoparticles, making them easier to handle. The photocatalytic activity of nanofibers can be improved by forming the nanofibers so that pores or cavities are formed.

[0069] [Note 7] The nanofibers of the polycrystalline ferrite material have an average diameter of 1 nm to 1000 nm, and the ratio of the average length to the average diameter (average length / average diameter) is 100 or more, as described in Appendix 6 of the photocatalyst. The nanofibers contained in the photocatalyst described in Appendix 7 have an average diameter and an average length / average diameter ratio within the above range, so a nonwoven fabric with high strength can be formed.

[0070] [Note 8] The nanofibers of the polycrystalline ferrite material are photocatalysts as described in Appendix 6 or 7, in which pores and / or cavities are observed by surface observation using SEM. The nanofibers contained in the photocatalyst described in Appendix 8 have a large surface irregularity and a large surface area. Therefore, the photocatalyst described in Appendix 8 exhibits even greater activity.

[0071] [Note 9] A photocatalyst described in any one of the appendices 6 to 8, having multiple nanofibers of polycrystalline ferrite material, in which the multiple nanofibers are intertwined to form a nonwoven fabric. The polycrystalline ferrite material contained in the photocatalyst described in appendice 9 has high shape stability because it forms a nonwoven fabric. Therefore, the photocatalyst described in appendice 9 exhibits good activity over an even longer period of time.

[0072] [Note 10] A method for producing a photocatalyst as described in any one of the appendices 1 to 9, comprising: a first step of preparing a polycrystalline ferrite material; a second step of annealing the prepared polycrystalline ferrite material; and a third step of applying an electric field to the annealed polycrystalline ferrite material. According to the method for producing a photocatalyst as described in appendice 10, since the polycrystalline ferrite material is annealed to improve its crystallinity before the electric field is applied, the resulting polycrystalline ferrite material has high polarity. Therefore, a photocatalyst exhibiting good activity can be produced industrially advantageously.

[0073] [Note 11] The method for applying the electric field in the third step is a corona discharge treatment method, as described in Appendix 10 of the method for manufacturing a photocatalyst. According to the method for manufacturing a photocatalyst described in Appendix 11, since the electric field is applied by a corona discharge treatment method, the photocatalyst can be polarized without being dispersed in the resin, so local dielectric breakdown is less likely to occur when the electric field is applied, and the photocatalyst can be uniformly polarized. In addition, the corona discharge treatment method is simple.

[0074] [Note 12] The method for producing a photocatalyst according to Appendix 10 or 11, wherein the polycrystalline ferrite material is nanoparticles, and the first step is to produce nanoparticles of the polycrystalline ferrite material by a solvothermal method. According to the method for producing a photocatalyst according to Appendix 12, since a solvothermal method is used to produce nanoparticles, nanoparticles with a stable size can be produced.

[0075] [Note 13] The method for producing a photocatalyst as described in Appendix 10 or 11, wherein the polycrystalline ferrite material is a nanofiber, and the first step is to produce a nanofiber of the polycrystalline ferrite material by electrospinning. According to the method for producing a photocatalyst as described in Appendix 13, since electrospinning is used to produce the nanofiber, nanofibers with a stable size can be produced.

[0076] [Note 14] The method for producing a photocatalyst as described in Appendix 13, wherein the step of producing nanofibers of polycrystalline ferrite material by electrospinning is to obtain nanofibers of polycrystalline ferrite material by electrospinning using a raw material solution containing a precursor of polycrystalline ferrite material and a binder resin. According to the method for producing a photocatalyst as described in Appendix 14, since a raw material solution containing a precursor of polycrystalline ferrite material and a binder resin is used, the density of the obtained nanofibers and the proportion of surface pores and / or cavities can be adjusted.

[0077] [Note 15] A method for decomposing pollutants, comprising the steps of: decomposing pollutants by contacting contaminated water containing pollutants with photocatalysts described in Appendix 1 to 9 under a light-irradiated environment; and separating the contaminated water from the photocatalysts. The method for decomposing pollutants described in Appendix 15 has a high pollutant decomposition capacity because it uses the above-mentioned photocatalysts.

[0078] [Note 16] The photocatalyst is a nanoparticle, and the step of decomposing the pollutant is carried out by dispersing the photocatalyst in contaminated water, and the step of separating the contaminated water from the photocatalyst is carried out by a solid-liquid separation method, as described in Appendix 15. The nanoparticles used in the pollutant decomposition method of Appendix 16 are fine and have a large surface area. Therefore, the pollutant decomposition ability is higher according to the pollutant decomposition method of Appendix 16.

[0079] [Note 17] The photocatalyst is a nonwoven fabric in which multiple nanofibers are intertwined, and the steps of decomposing pollutants and separating the contaminated water from the photocatalyst are performed by passing the contaminated water through the nonwoven fabric, as described in Appendix 15. The photocatalyst used in the pollutant decomposition method of Appendix 17 does not easily release nanofibers even when contaminated water is passed through it. Therefore, the pollutant decomposition method of Appendix 17 has a low environmental impact.

[0080] [Note 18] The nonwoven fabric is composed of multiple nanofibers intertwined with each other, and the nanofibers contain a polycrystalline ferrite material, which has a coercive field Ec. Because the nonwoven fabric described in Appendix 18 is polarized, excited species such as hydroxyl radicals are easily generated, and the nanofibers are less likely to leak out even when contaminated water is passed through it. Therefore, the nonwoven fabric described in Appendix 18 is useful as a decomposition and removal agent for pollutants contained in contaminated water.

[0081] [Note 19] An information processing device that transmits at least photocatalytic information relating to the photocatalyst, ferroelectric-related information relating to the coelectric field Ec, and photocatalytic activity-related information indicating photocatalytic activity based on the relationship between the photocatalyst-related information and the ferroelectric-related information. According to the information processing device described in Appendix 19, the technical significance of photocatalysts utilizing ferroelectricity can be accurately communicated to users of photocatalysts.

Claims

1. A photocatalyst comprising a polycrystalline ferrite material, The aforementioned polycrystalline ferrite material is a photocatalyst having a coelectric field Ec of 0.1 MV / m or more and 2.0 MV / m or less.

2. The photocatalyst according to claim 1, wherein the polycrystalline ferrite material has a perovskite crystal structure.

3. The photocatalyst according to claim 1 or 2, wherein the polycrystalline ferrite material is a nanoparticle.

4. The photocatalyst according to claim 1 or 2, wherein the polycrystalline ferrite material is a nanofiber.

5. The first step is to prepare the polycrystalline ferrite material, The second step is to anneal the aforementioned polycrystalline ferrite material, A method for producing a photocatalyst, comprising a third step of applying an electric field to the polycrystalline ferrite material.

6. The method for producing a photocatalyst according to claim 5, wherein the method for applying the electric field in the second step is a corona discharge treatment method.

7. The aforementioned polycrystalline ferrite material is a nanoparticle, The method for producing a photocatalyst according to claim 5 or 6, wherein the first step is to produce nanoparticles of the polycrystalline ferrite material by a solvothermal method.

8. The aforementioned polycrystalline ferrite material is a nanofiber, The method for producing a photocatalyst according to claim 5 or 6, wherein the first step is to produce nanofibers of the polycrystalline ferrite material by electrospinning.

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